As is known in the art, in many applications it is required to provide a plurality of microwave transmission lines to electrically interconnect electrical devices. One such application is in interconnecting electrical devices formed as a Monolithic Microwave Integrated Circuit (MIMIC) as shown in FIG. 1. The microwave transmission lines may be: a plurality of coplanar waveguide transmission lines, as shown in FIGS. 1A′ and 1A″; and where the signal strip conductor (FIG. 1A″) is disposed between a pair of ground strip conductors (FIG. 1A″) all formed of the upper surface of a dielectric (FIG. 1A′) for a monolithic microwave integrated circuit (MMIC) and the electric field, here represented by the arrow, vector, (e) (FIG. 1A′) is between the signal strip conductor and the pair of ground strip conductors; or a plurality of microstrip transmission lines, as shown in FIGS. 1B′ and 1B″, where a signal strip conductor (FIG. 1B″) on an upper surface of a dielectric is separated by an underlying ground plane conductor (FIG. 1B′) on a bottom surface of the dielectric and the electric field, e, as shown in FIG. 1B′, is through the dielectric between the signal strip conductor and the dielectric.
In order to maximize the utilization of the surface of the MMIC for various active and passive devices used in the MMIC and interconnected by these microwave transmission lines it is generally required that the spacing, X, (FIG. 1) between these microwave transmission lines be minimized yet still have these proximate transmission lines electrically shielded one from another.
One technique suggested to improve isolation between a pair of microstrip transmission lines is described in a paper entitled “Improved Electrical Performance of Interconnects Using Inkjet Printing” by S. Huang, K. Xiao and X. Ye, 2016 IEEE International Symposium on Electromagnetic Compatibility (EMC), Ottawa, ON, 2016, pp. 256-260 shown in FIG. 2A. Here absorbing material is printed over each of the strip conductors and between the pair of strip conductors. A technique used to improve isolation for a pair of CPW transmission lines is shown in FIG. 2B. Here a pair of CPW transmission lines each has a signal strip conductor (S) disposed between a pair of ground strip conductors (G) on the upper surface of a dielectric substrate. Here, each one of the CPW transmission lines includes electrically conductive vias (V) passing through the dielectric to electrically connect the ground strip conductors (G) to a ground plane conductor (GP) on the bottom of the dielectric substrate. The shielding between the pair of CPW transmission lines is provided by bond wires or ribbons to form wire bonds (WB) suspended over the signal strip conductor having ends thereof bonded to the pair of ground strip conductors and spaced apart less than typically every ⅛ wavelength of the nominal operating wavelength of the microwave transmission line structures to form a Radio Frequency (RF) cage, as shown.